Abstract: Acute toxicity of diafenthiuron emulsifiable concentrate（EC） and diafenthiuron suspension concentrate（SC） to aquatic organisms was studied.The results show that 25% diafenthiuron EC was classified to be high in toxicity to Brachydanio rerio and Macrobrachium nippoensis and low to Eriocheir sinensis;whereas 50% diafenthiuron SC was classified to be high in toxicity to Brachydanio rerio,moderate to Macrobrachium nippoensis and low to Eriocheir sinensis.The multi-tiered ecological risk assessment method was used to assess the risk of diafenthiuron to aquatic organisms in a simulated paddy field-fish pond ecosystem.Results show that 25% diafenthiuron EC posed an extremely high risk to Brachydanio rerio and Macrobrachium nippoensis,but a low risk to Eriocheir sinensis;whereas 50% diafenthiuron SC did a high risk to Brachydanio rerio,and a relatively low risk to Macrobrachium nippoensis and Eriocheir sinensis.In view of its high risk potential to aquatic organisms,for protection of environment and organisms the use of diafenthiuron should be banned or restricted in paddy fields.

Key words: diafenthiuron, aquatic organisms, Brachydanio rerio, Macrobrachium nippoensis, Eriocheir sinensis, toxicity, simulated paddy field-fish pond ecosystem, ecological risk assessment